The integrin family of cell surface receptors serve cellular adhesion functions. The receptors form a link between the extracellular matrix and the cytoskeleton through their binding to various extracellular components. Each integrin receptor is a heterodimer comprised of an .alpha. and a .beta. subunit. At least 11 .alpha. chains (Ruoslahti and Giancotti, 1989) and six .beta. chains (Sheppard et al., 1990) have been recognized in man. Each .alpha. subunit tends to associate with only one type of .beta. subunit, but there are several exceptions to this rule (Hemler et al., 1989; Cheresh et al., 1989; Holzmann et al., 1989; Freed et al., 1989).
The human heterodimer VLA-6 was identified using the monoclonal antibody GoH3, which is immunoreactive with the .alpha..sub.6 subunit expressed on the surface of mouse and human cells. Hemler et al. J. Biol. Chem., 263:7660-7665, (1988); and Sonnenberg et al. J. Biol. Chem., 262:10376-10383, (1987). The amino terminal sequence of the human VLA-6 .alpha..sub.6 subunit was determined from purified protein (Kajiji et al. EMBO J. 8:673-680,1989) and was used to design degenerate oligonucleotides for probing a cDNA library. The full length sequence of .alpha..sub.6 cDNA, and its predicted amino acid sequence, were elucidated subsequent to cDNA cloning. Tamura, et al., J. Cell Biol., 111:1593-1604 (1990). While Tamura et al., supra, also disclose multiple cDNA sequences encoding the VLA-6 .beta..sub.4 subunit, there is provided no evidence that additional VLA-6 .alpha..sub.6 subunits exist. European Patent Application Publication Number 279,669 (published Jul. 24, 1988) describes human .alpha..sub.6 and .beta..sub.4 subunits of an integrin receptor and the complex they associate to form on pancreatic and other cancer cells. The publication does not describe or suggest that an isoform of the .alpha..sub.6 subunit exits.
The full length sequence of a hamster cDNA encoding the Gap b3 cell surface membrane glycoprotein was described by Tsuji et al., J. Biol. Chem., 265:7016-7021 (1990). Based on the predicted amino acid sequence and predicted overall structure, it was suggested that Gap b3 is the hamster homolog of the .alpha..sub.3 integrin subunit. The sequence of a cDNA encoding the partial sequence of chicken .alpha..sub.3 protein was disclosed in Hynes et al. J. Cell Biol., 109:409-420 (1989). The cytoplasmic regions of these clones do not share homology with the cytoplasmic region of .alpha..sub.3B disclosed herein, and are therefore assumed to encode .alpha..sub.3A subunit isoform. Furthermore, neither publication suggest the possibility of an .alpha..sub.3B subunit.
The N-terminal amino acid sequence of human .alpha..sub.3 protein is provided in European Patent Application Publication Number 330,506 (published Jul. 3, 1989). That publication provides no suggestion that an isoform of the .alpha..sub.3 protein, namely .alpha..sub.3B, exists.